1. Field of the Invention
The present invention is broadly concerned with improved methods and apparatus for the drying/cooling of fracturing sands used in petroleum rock formations. More particularly, the invention is concerned with such methods and apparatus wherein use is made of an induced-draft, co-current, single-pass rotary dryer equipped with an additional wet sand input device situated between the primary wet sand inlet at one end of the dryer, and a dried sand outlet at the other end thereof. The additional wet sand added to the dryer serves to cool the initially introduced dried sand while simultaneously drying the additional wet sand by evaporation. The final conditioned product from the rotary dryer has suitable moisture and temperature levels for downstream sizing.
2. Description of the Prior Art
Some subsurface rock formations, such as organic shales, contain large amounts of oil, natural gas, or natural gas liquids that will not flow freely to a well because the rock formations either lack permeability, or the pore spaces are so small that these fluids cannot readily flow through them. The hydraulic fracturing process addresses these problems by generating fractures in the rock formations. This is done by drilling a well into the rock, sealing the portion of the well in the petroleum-bearing zone, and pumping water under high pressure into that well zone. This water is generally treated with chemicals and thickeners to create a viscous gel, which suspends grains of fracturing sands. Large pumps are used to increase the water pressure until it is high enough to exceed the breaking point of the rock formations. When this breaking point is reached, the formations fracture suddenly and water rushes into the fractures, inflating them and extending them deeper into the rock. When the pumps are deactivated, the fractures deflate, but do not close completely, because they are propped open by billions of grains of the fracturing sand. The new fractures in the rock, propped open by the sand grains, form a network of pore spaces that allow petroleum fluids to flow out of the rock and into the well. Thus, fracturing sand is also known as a “proppant,” because it props the rock fractures open.
Fracturing sand, generally referred to in the art as “frac sand,” is desirably a high-purity quartz sand with very durable and round grains. Most frac sand is a natural material derived from high-purity sandstone. The demand for frac sand has exploded in the past several years as thousands of oil and natural gas wells are being stimulated using the hydraulic fracturing process. A hydraulic fracturing job on a single well may require several thousand tons of sand. Accordingly, a substantial frac sand industry has developed in the past few years.
Frac sand products must meet very demanding specifications in order to be used in fracturing operations. The sand grains should be substantially spherical in shape, have size specifications matched to particular job applications, and be highly durable to resist crushing. Such sands may be dredged or mined from naturally occurring sources, especially in Wisconsin and Texas. However, frac sand cannot be used straight from the ground, and it must be subjected to conditioning in specialized frac sand plants. In such facilities, the native sand is first washed in a “wet plant,” where mud and slimes are separated, along with very fine sand grains. After wet plant treatment, the clean sand has a moisture content of approximately 6-7% by weight, and cannot be screened or otherwise size-classified in this condition. Therefore, the wet sand must be dried to a relatively low moisture content on the order of 0.5% by weight in order to permit sizing. Moreover, the hot, dried sand must be cooled before sizing, in order to prevent damage to downstream equipment.
A variety of equipment has been employed in the past for the drying and cooling of wet frac sand. These are generally referred to as fluid bed dryers (both static and vibratory), and rotary dryers of counter-current or co-current design. A known rotary dryer may include integrated cooling features, where incoming sand is dried in an inner pass of the dryer and is cooled in an outer pass. It has also been known to add wet sand to the dried but not yet cool sand in the aforementioned multiple-pass rotary dryer, so that cooling is enhanced by evaporation of water from the moist sand proportion. However, such a dryer/cooler has very high horsepower requirements (e.g., 200 HP), and therefore equipment and utility costs become significant.
Fluid bed dryers have a number of significant disadvantages. They are optimally suited only for fine-grain materials having a diameter of about 4-6 mm, and have only limited drying air temperature ranges. Such units are also sensitive to abrupt changes in solid material particle sizes, moisture content, throughput rates, and periodic cutouts of drying air. Moreover, this type of equipment has relatively high electrical energy requirements, and expensive air systems consisting of fans, ducts, and separate hot gas generator equipment. Consequently, significant efforts and expenses are involved in commissioning fluid bed dryer systems for parameter optimization. Fluid bed dryer systems are necessarily light-weight designs for ease of startup on vibratory or shaker models, and require high differential pressures to overcome higher fan compressions and horsepowers. In many systems, stainless steel chambers or perforated troughs may be required for heat tolerance.
Static bed dryers may require refractory lined gas chambers and have high air volume requirements owing to limited hot gas requirements. The low temperature cooling air which is captured during operation can be near dew point depression levels, resulting in baghouse plugging.
Generally speaking, drum dryer systems have a number of advantages, including low heat energy requirements even when drying only partial loads, by simple adjustment of exhaust air volumes. Also, it is usually not necessary to adjust the air volumes during product change-overs. The air exhaust equipment from the drum dryer is comparatively simple inasmuch as air is extracted from only one point on the dryer. Consequently, drum dryer systems are simple to install and commission, tolerant to operating faults, very rugged with long service lifetimes, and have low wear and replacement part requirements.
Rotary dryer systems may be either counter-current or co-current design. Counter-current systems have a number of disadvantages. Since there is no relationship between the exhaust gap temperature, the burner must be controlled by the temperature of the material exiting the dryer. However, change in process conditions is based upon the incoming material, not the exiting material. Therefore, the response of the burner is not known for several minutes until the dryer has cycled through the established residence time for the material. Further, since the feed material is entering on the cool, wet end of the dryer, a quick flash of evaporation usually occurs in the middle section of the drum instead of near the out-feed end. It is therefore not unusual for a cake ring to form on the shelf just prior to the quick-flash location. Given the limited control of the discharge gas temperature in these systems, there is a real danger that the exhaust gap temperature will drop below the dew point, especially in the winter. This increases the risk of mudding of the bag filters in the dust collector. Since the control of countercurrent systems has a long lag time, most operators tend to over-dry and over-heat the incoming material so that the system will run more smoothly. In contrast, co-current dryer designs rely upon the exhaust gas temperature for control, because the gas molecules travel through the dryer in seconds, instead of the minutes required for product travel time.
There is accordingly a need in the art for more efficient equipment and methods for the drying and cooling of wet frac sand, than has heretofore been available.